Sexual Plant Reproduction 4(2): 110-112 (Apr 1991)
Influence of the pH of the stigmatic exudate on male-female interaction in Rosa hybrida L
S. Gudin, L. Arene

Summary

The in vitro germination of rose pollen is influenced by the pH of the medium. Both germination percentage and length of emitted pollen tubes were maximal for in vitro germination and tube elongation at pH 5 and minimal at pH 3 and 9 (Rosa hybrida L. var P 30 pollen). Three varieties characterized by having a stigmatic exudate of pH = 5 and another three varieties having one of pH = 9 were pollinated with the same pollen. Pollination effectiveness, as indicated by hip set ((number of hips/pollinated flowers) x 100) and mean number of achenes per hip, were significantly different: it was much higher for the varieties with the stigmatic exudate of pH = 5. pH control on pollination efficiency and subsequent fecundation success is proposed and discussed.

Introduction

Pollen germination and pollen-tube elongation in female tissues depend on a complex set of "recognition factors" that interact at the stigmatic level (Dumas and Gaude 1981; Dumas et al. 1984). Mulcahy (1974, 1979) and Mulcahy and Mulcahy (1983) suggest that styles and stigmas in angiosperms represent selection places for male gametophytes. The stigmatic secretion or "exudate" which characterizes so-called "wet stigmas" (a classification criterion first defined by Burck in 1901 and still used — see Heslop-Harrison 1981 and Dumas et al. 1988) seems to be an important agent in this selection. The exudate is extruded during a period "more or less connected with stigma receptivity" (Dumas et al. 1988). The exudate probably possesses some of the receptor sites for pollen recognition at the stigma level (see Dumas et al. 1984), and contributes to wall formation of the pollen tube (Loewus and Labarca 1973). The stigma is known to have cytochemical activities (mostly enzyme activities) that lead to the completion of pollen adhesion and hydration (Knox 1984). Some of these activities and physiological events are, since the pioneer work of Konar and Linskens (1966), often studied with the help of artificial stigmas (see Bowman 1984; Knox et al. 1986).

From in vitro studies performed on artificial media, it is well known that pH modifies germination percentages and emitted pollen tube lengths (Therios et al. 1985) of numerous plant species (Sisa 1930; Kwack 1965). To our knowledge, the possible control by the pH of the stigmatic exudate over in vivo fecundation success has never been investigated.

Materials and methods

Plant material

Three varieties historically known by the breeder for their high degree of female fertility were chosen (R. hybridia L. var 364-73.A, R. hybrida L. var 'Meitulandi' and R. hybrida L. var 'Meiringa'). Three others known for their low fertility (R. hybrida L. var 272- 69.A, R. hybrida L. vat 'Jelrafloki' and R. hybrida L. var- 'Meisadina') were also chosen, All six varieties were grown in the same greenhouse in Antibes. Pollen of Rosa hybrida L. var. P 30 was used in this study.

Pollination experiments

Pollen collection, female flower preparation, and pollination procedures have already been described (Gudin et al. 1991). Six different crosses were made on the same day in October 1987. The number of flowers pollinated for each "female" variety was as follows: 364-73.A. (15), 'Meitulandi' (80), 'Meiringa' (73), 272-69.A (71), 'Jelrafloki' (15), and 'Meisadina' (20). Four months after pollination, the hips were collected and counted, as were the number of achenes per hip.

Before making each of the six crosses, the pH of the stigmatic exudate on five flowers (used solely for this purpose) of each female variety was measured. This was done by rubbing the three areas of a Dosatest paper strip (pH 4.%10.0) against the stigmatic surface. At the same time, an in vitro germination test of the pollen was done on a basal medium (Gudin et al, 1991) adjusted to pH of 3, 5, 7, or 9 with 0.1 N NaOH or HC1. The mean number, calculated from 50 pollen grains, that germinated and the mean length of emitted pollen tubes were observed according to Gudin et al. (1991).

Results

Figures 1 and 2 show the influence of pH on in vitro germination of pollen of R. hybrida L. var. P 30. The number of in vitro-germinated grains can be seen to be significantly lower at pH 3 and 9 than at pH 5 and 7 (Fig. 1). Values of 5 and 7 were the most favorable for germination, whereas the least favorable were pH values of 3 and 9.

The length of the emitted pollen tubes was maximal at pH 5, when it was twice the length observed at pH 3. The shortest tubes occurred at pH 9 (Fig. 2).

The varieties used as females can be split into two groups: one is characterized by an acid stigmatic pH of 5 and the other has a basic pH of 9 (Table 1). For each variety, there was no variation in stigmatic pH among the five flowers tested. Furthermore, this pH did not vary when observed across a l-week span. The hip sets of all three varieties with a stigmatic pH of 9 were all lower, two being significantly so, than those of any of the three varieties with a stigmatic pH of 5 (Table 1). The three varieties with low stigmatic pH gave achene numbers per hip that on average were nine times larger than those of the three varieties with a high stigmatic pH.

 

pH 5   pH 9
364-73.A   272-69.A
Meitulandi [Hidalgo]   JELrafloki [Sangria?]
Meiringa [Carte Blanche]   Meisadina [Golestan]

As a possible explanation of the low germination at pH 9, we suggest that at a high pH borate ion accumulates and reaches toxic levels as a result of boric acid dissociation (Kim et al. 1985).


References